Aside from the Funny Lady poster in The Gumball Rally, there are some other Barbra connections.
One of the cars in the coast-to-coast race is a red Ferrari convertible. At one point, the chase takes place in Arizona. The scene reminded me of A Star is Born. O.K. not much coincidence there.
This movie came out in late July/early August 1976 (depending on which source to reference), a few months before A Star is Born. They could have conceivably been filmed around the same time. Again, not much of a coincidence there.
Both films were from the same studio, Warner Brothers, and the same production company, First Artists. A-ha. Is it possible that they used the same car for both films?
I did a little more digging on the internet and discovered this from imdb.com about The Gumball Rally:
Two Ferrari Daytonas were used (main car and a backup) The serial numbers were 14829 and 16467. Chassis 14829 is currently at the Peterson Automotive Museum and was once owned by 'Mel Blanc.' Chassis 16467 also appeared in 'A Star in Born' (1976)!
Also, Michael Sarrazin and Gary Busey star in the film, along with a supporting role by Colleen Camp (Funny Lady).
Even with all these Barbra connections, I couldn't give this film a ringing endorsement unless you're into car racing films.